In conventional X-ray imaging, contrast in an image is usually achieved by an attenuation of X-ray radiation in an object to be imaged. Over the past decade, several techniques have been developed to exploit the contrast given by a phase-shift of X-ray radiation transmitted through the object. X-ray photons of the X-ray radiation may be absorbed, coherently refracted and/or scattered. Whether the X-ray photons are absorbed, coherently refracted or scattered, the respective interaction may be used to form a respective image or an image component of a multi-component image. To acquire this information, a dark field imaging system may be used. Such a system comprises a source for generating X-ray radiation, a detector for detecting X-ray radiation and an interferometer arranged between the source and the detector for creating interference pattern. The interferometer may be formed by a Talbot grating interferometer. Such an interferometer may comprise an absorbing source grating, a phase grating and an analyzer grating. The gratings are arranged one after the other in an X-ray beam path between the source and the detector. Further, an object receiving space is arranged between the source and the detector, in particular between two of the gratings of the interferometer. The phase grating may be formed to introduce a phase-shift of an incoming X-ray beam (formed by X-ray radiation of the source) which creates an interference pattern behind the phase grating.
Furthermore, it is to be considered that an X-ray beam imposed on a subject, in particular a human subject, positioned at the object receiving space may cause an attenuation, a scattering and/or a refraction of the X-ray beam. In principle, the dark field imaging system may be configured to acquire image data via the detector representing an attenuation, a scattering and/or refraction caused by the subject. The refraction caused by the subject may be determined based on the displacement of the interference fringes. However, as the interference fringes may not be spatially resolved with a conventional X-ray detector, the acquiring of measurement data may be performed by using a phase-stepping technique. In this context, the X-ray radiation intensity oscillation behind the analyzer grating during a lateral stepping scan of one of the gratings is recorded and the fringe displacement is determined in terms of the phase-shift of the oscillation curve of each of the plurality of detector pixels of the X-ray detector. The measured phase-shift of the intensity oscillation in each detector pixel may relate to the local refraction angle, a distance between the phase grating and the analyzer grating, and a period of the analyzer grating. In this context, reference is made to the document “Inverse geometry of grating-based X-ray phase-contrast imaging”, Journal of Applied Physics 106, 054703 (2009).
Based on the detector signal provided by the X-ray detector, three images may be determined representing an object to be imaged. The object is preferably a human subject. The first image may be referred to as the conventional image or the attenuation image. The conventional image may be determined based on a component of the detector signal representing the attenuation imposed on the X-ray radiation transmitted through the object. The attenuation imposed on an X-ray beam may also be referred to as an attenuation disturbance caused to an X-ray beam. The second image may be referred to as the dark field image. The X-ray detector signal may comprise a component representing a scattering imposed on the X-ray radiation transmitted through the object. As the scattering may cause de-coherence to the respective X-ray beam, the scattering may also be referred to as a de-coherence disturbance caused to an X-ray beam. The dark field image may be determined based on a component of the detector signal representing the de-coherence imposed on the X-ray radiation transmitted through the object. The third image may be referred to as the differential phase contrast image. The X-ray detector signal may comprise a component representing a refraction imposed on the X-ray radiation transmitted through the object. The differential phase contrast image may be determined based on a component of the detector signal representing the refraction imposed on the X-ray radiation transmitted through the object. As a result, three images may be determined, namely a differential phase-contrast image based on the refraction imposed on the X-ray beam, a dark field image based on the scattering imposed on the X-ray beam and a conventional image based on the attenuation imposed on the X-ray beam.
Document WO 2015/180977 A1 discloses a phantom body for the use in a phase-contrast imaging system for calibrating the phase-contrast imaging system. The phantom body comprises three mutually distinct and separately arranged parts. A first part of the phantom device is configured to cause a phase-shift disturbance. A second part of the phantom device is configured to cause an absorption disturbance. A third part of the phantom body is configured to cause a de-coherence disturbance. Each of said disturbances relates to an X-ray beam, when said X-ray beam passes through the phantom body. As a result, each of the parts of the phantom body exclusively responds to exactly one of the respective three disturbance effects. Thus, the phantom body allows to calibrate the phase-contrast imaging system, in particular with respect to three different images to be acquired, namely the conventional image, the differential phase-contrast image and the dark field image.